A hierarchy is a system of nested groups. A standard organizational chart is a hierarchy, with employees grouped into business units and departments reporting to a centralized authority. Other kinds of hierarchies include government bureaucracies, biological taxonomies, and a system of menus in a software program. Hierarchies are inherently “top-down,” in that they are designed to enable centralized control from a single, privileged position.
A network, by contrast, has no “correct” orientation, and thus no bottom and no top. Each individual, or “node,” in a network functions autonomously, negotiating its own relationships and coalescing into groups. Examples of networks include a flock of birds, the World Wide Web, and the social ties in a neighborhood. Networks are inherently “bottom-up,” in that the structure emerges organically from small interactions without direction from a central authority.